1. Field of the Invention
The present invention refers to a bootstrap circuit in DC/DC static converters, particularly in DC/DC static converters in step-down configuration comprising such bootstrap circuit. The present invention finds particular application in static converters for circuits realized in monolithic form.
2. Description of Related Art
DC/DC static converters are widely used in power supplies, actuator systems, displays, signal processing systems etc. and are based on well-known technologies in which a magnetic means, such as a transformer or an inductance, is driven by at least one power switch. Said switches are controlled by a Pulse Width Modulation (PWM) system commutating at a certain frequency set by a system timing signal.
In the field of static converters there are various typologies, such as step-down converters, also known as xe2x80x9cbuckxe2x80x9d converters in which the regulated output voltage is less than the input voltage, step-up converters, also known as xe2x80x9cboostxe2x80x9d converters in which the regulated output voltage is greater than the input voltage and the converters called xe2x80x9cbuck-boostxe2x80x9d in which the regulated output voltage has an inverse sign compared to the input voltage.
In the configuration of the step-down converters, illustrated in FIG. 1, the power switch is represented by an N-channel DMOS transistor in high-side configuration that is a configuration in which the source terminal is floating and the magnetic means is represented by an inductance L. The source voltage of said DMOS can vary, therefore, between Vin, that is the input voltage of the converter when the DMOS is on and forces current in said inductance L, and xe2x88x92Vd, which is the voltage drop on a recirculation diode when the DMOS is off and the current stored in L flows through said diode. A graph is shown in FIG. 3 representing the circuit of FIG. 1 when the charge connected to it is at the maximum value.
As is well known to a technician in the sector, in order that an N-channel DMOS transistor is well on, that is its channel resistance Rds(ON) is minimized, it is necessary for the voltage difference between the gate terminal and the source terminal of said DMOS to be greater than about 10V. Nevertheless when the DMOS is well on, that is when it operates in deep ohmic region, the voltage of the source terminal is about equal to the input voltage, apart from the voltage drop due to the resistance of the DMOS itself. This implies the necessity to have a voltage available which exceeds the input voltage that is wanted to make the DMOS work in deep ohmic region. A widely diffused technique to obtain this boosting is the so-called bootstrap technique.
Nevertheless, also in circuit configurations comprising bootstrap means, such as capacitors, inductors and recirculation diodes, there is the disadvantage that the DMOS transistor is driven efficiently and its Rds(ON) is minimum while said bootstrap means guarantee a sufficiently higher voltage than the input voltage.
The limit of the above mentioned bootstrap circuits is that the DMOS cannot be kept on for an indefinitely long time and above all a minimum time has to be guaranteed during which the magnetic means, that is the inductor, degausses. In this period of time the potential of the source terminal must therefore be sufficiently near to zero so that the capacitor recharges.
The need to recharge the capacitor is a pressing problem from the technological point of view when the DC/DC converter functions with loads of modest value because one of the following disadvantages can occur:
a) having a load of modest value the DMOS xe2x80x9conxe2x80x9d time is very short and also the recirculation diode recharge time is short as is shown in FIG. 4. This means that if the load is sufficiently small the recirculation time of the diode can become so short that it does not enable the capacitor to recharge.
b) the DMOS must be on for a minimum time in function of the delays of the control circuit. When the load is very low and requires the DMOS to be on for a shorter time than the minimum time, not being able to satisfy such condition, in the short time there is an excess of energy carried on the load with the consequent slight increase of the output voltage. The control loop in feedback reacts and several commutation cycles are skipped so as to bring the output voltage to the regulation value and re-establish the correct energy balance. In this case, therefore, the time available for recharging the bootstrap capacitor diminishes even further.
c) if the input and output voltages are relatively high (both exceeding around ten Volts) and near each other the voltage forced in the inductor during the time in which the DMOS is on can be so small that the demagnetization of the inductor comes about at the expense of the energy of the capacitance of the source terminal. When this occurs the voltage of the source terminal remains several Volts above zero and the recirculation diode is not switched on, as described in FIG. 5.
d) during the DC/DC converter turning off phase if the output voltage is relatively high (exceeding around ten Volts) the input voltage will diminish very slowly and when it approaches the output voltage it falls back into condition (c). The output voltage does not go to zero monotonically but oscillating.
The result of the previously described phenomena is that the capacitor progressively discharges and as soon as its voltage goes below the threshold voltage of the DMOS transistor, said transistor cannot be switched on again and the converter will be blocked. According to the value of the input and output voltage and of the values of the components constituting the converter, intermittent functioning or a definitive block of the same converter will be obtained.
The U.S. Pat. No. 5,627,460 tells how to use the technique of the so-called xe2x80x9csynchronized diodexe2x80x9d applied to a DC/DC converter step-down in which the recirculation diode is replaced by an N-channel DMOS in low-side configuration that results being driven in push-pull in respect of the main DMOS. Such a solution is not applicable to the standard typology of the circuits because the latter provides for only one recirculation diode and not two diodes synchronized in push-pull.
The International Rectifier in one of its applicative notes (DT94-1A xe2x80x9cKeeping the bootstrap capacitor charged in buck convertsxe2x80x9d) relating to its device IR2125, suggests to apply a resistor and a Zener diode. This technique is effective when the input voltage is quite higher than the output voltage and therefore not applicable in case (c). In addition if the input voltage has a wide variation interval, to be able to guarantee sufficient current to the minimum input voltage a relatively low resistive value is necessary which leads, when the input voltage is high, to high power dissipation in the resistance and in the Zener.
Other known solutions resort to magnetic means but have the disadvantage that when the load of the converter has a modest value the magnetic energy is also very low, comparable to or lower than that dissipated by the effect of the not-ideal coupling between the windings with consequent efficiency losses.
In view of the state of the technique described, the object of the present invention is to realize a circuit suitable for avoiding the discharging of the bootstrap capacitor so as to permit more effective driving of the DMOS in high-side configuration.
In accordance with the present invention, said object is reached by means of a bootstrap circuit in DC/DC static converters characterized in that it comprises a fixed frequency signal, a recharge circuit of a capacitor and current generator means, said generator means controlled so as to emit current pulses, in synchrony with said fixed frequency signal, of a predetermined duration, every time the charge accumulated by said capacitor goes below a predetermined level.
In addition said object is also reached by means of a bootstrap circuit in DC/DC static converters, characterized in that it comprises a comparator suitable for comparing the charge degree of said capacitor with a predetermined value so as to compensate the discharge of said capacitor.
Thanks to the present invention it is possible to realize a circuit capable of eliminating the problems connected to the bootstrap technique.
In addition it is possible to realize a circuit that permits the turning on of a DMOS high-side for an undetermined time.
The present invention finds application also in other typologies of static converters such as the xe2x80x9cbuck-boostxe2x80x9d, the xe2x80x9cflybackxe2x80x9d, the double switch xe2x80x9cforwardxe2x80x9d, the half bridge and the full bridge, that is circuits used for the control of motors.
In addition the present invention finds particular application in monolithic static converters, that is those converters in which both the control part and the power switch are integrated into the same chip.